1. Field of the Invention
The present invention relates to an orthogonal polarization multiplexing transmission apparatus and a multiplexing method used for the apparatus, and in particular, to the orthogonal polarization multiplexing transmission apparatus used for orthogonal polarization multiplexing and transmission dispersion compensation in DWDM (dense wavelength division multiplexing) and a multiplexing method used for the apparatus.
2. Description of the Related Art
FIG. 18 is a waveform chart of an example of wavelength division multiplexing (WDM). This diagram shows a condition in which light signals of channels 1 to 5 are multiplexed in increasing order of wavelength. Moreover, the vertical axis indicates transmission power Pw (W: watt), and the horizontal axis indicates a wavelength λ (nm). This is the same as to the drawings referred to hereafter. In the past, 64-wave WDM (0.4-nm spacing) was used as an example, and a wavelength spacing of each light signal is relatively wide as shown in this diagram.
On the other hand, in recent years, dense wavelength multiplexing (DWDM) such as 128-wave WDM (0.2-nm spacing) is considered because of requests to further increase transmission capacity. FIG. 19 is a waveform chart of an example of this dense wavelength division multiplexing. As shown in this chart, in the case of dense wavelength multiplexing, the wavelength spacing of each light signal is narrower than the waveform in FIG. 18. Therefore, crosstalk (interference) occurs between adjacent channels as shown in a crosstalk explanation diagram in FIG. 20, and it becomes a cause of signal deterioration.
Thus, orthogonal polarization multiplexing is used as one of the means for preventing this crosstalk. FIG. 21 is a waveform chart showing an example of the orthogonal polarization multiplexing. With reference to this chart, it is constituted so that odd-numbered array waves (1, 3, 5, 7, . . . ch) and even-numbered array waves (2, 4, 6, . . . ch) are mutually in the polarization directions of 90 degrees, that is, orthogonal. Thus, polarized waves of the adjacent channels are orthogonal, so that the crosstalk between the adjacent channels can be prevented.
On the other hand, the transmitted light signals of wavelength division multiplexing have wavelength dispersion occurring in an optical transmission line such as an optical fiber before they are received by a receiving apparatus. FIG. 22 is an explanatory diagram of the signal deterioration due to the wavelength dispersion. As shown in this diagram, a transmitting light signal T has the wavelength dispersion occurring in the optical transmission line, and consequently the waveform of a receiving light signal R in the receiving apparatus has its waveform collapsed compared to the transmitting light signal T. The longer the transmission line becomes, the more significant this waveform deterioration becomes due to influence of the wavelength dispersion. Binary determination of data becomes difficult in the receiving apparatus due to this waveform deterioration.
Thus, a dispersion-compensating fiber (DCF) is used in order to prevent the waveform deterioration caused by this wavelength dispersion. FIG. 23 is a block diagram of an example of the orthogonal polarization multiplexing transmission apparatus using the past dispersion-compensating fiber. With reference to this diagram, the example of the orthogonal polarization multiplexing transmission apparatus in the past is constituted by including an odd-number array polarization multiplexing portion 401, an even-number array polarization multiplexing portion 402, apolarization orthogonal multiplexer 17, an optical amplifier 18 and a dispersion-compensating fiber 19.
Next, operation of this orthogonal polarization multiplexing transmission apparatus will be described. An odd-numbered array multiple light signal outputted from the odd-number array polarization multiplexing portion 401 and an even-numbered array multiple light signal outputted from the even-number array polarization multiplexing portion 402 are multiplexed by the polarization orthogonal multiplexer 17 so that a polarization signal wherein an odd-numbered order and an even-numbered order are mutually orthogonal is generated. Next, an optical level lowered by insertion loss of the polarization orthogonal multiplexer 17 is amplified to a predetermined level by the optical amplifier 18, and the light signal after amplification has band dispersion compensation performed thereto by the dispersion-compensating fiber 19 and is outputted. Moreover, as shown in this diagram, an input route to the polarization orthogonal multiplexer 17 is a polarization preserving section, and an output route from the polarization orthogonal multiplexer 17 onward is a polarization non-preserving section.
Moreover, it is clear that, also in the orthogonal polarization multiplexing, the ideal is to individually perform dispersion compensation to each light signal, that is, to provide one dispersion-compensating fiber to each light signal input portion of the odd-number array polarization multiplexing. portion 401 and the even-number array polarization multiplexing portion 402 for instance. To do so, however, it is considered that the dispersion-compensating fiber for preserving a plane of polarization is necessary. Nevertheless, such a dispersion-compensating fiber has not been developed to date, and so a configuration wherein the dispersion-compensating fiber 19 is placed on an output side of the polarization orthogonal multiplexer 17 as in FIG. 23 is generally used.
Next, the example of the orthogonal polarization multiplexing transmission apparatus using the past dispersion-compensating fiber will be described further in detail. FIG. 24 is a detailed explanatory diagram of the orthogonal polarization multiplexing transmission apparatus using the past dispersion-compensating fiber. Moreover, the same components as in FIG. 23 are given the same numbers and description thereof will be omitted.
With reference to FIG. 24, the example of the orthogonal polarization multiplexing transmission apparatus in the past is constituted by including an orthogonal polarization multiplexing portion 400, the optical amplifier 18, the dispersion-compensating fiber 19 and an optical amplifier 20.
In addition, the orthogonal polarization multiplexing portion 400 is comprised of the odd-number array polarization multiplexing portion 401, the even-number array polarization multiplexing portion 402 and the polarization orthogonal multiplexer 17. The odd-number array polarization multiplexing portion 401 and the even-number array polarization multiplexing portion 402 are comprised of a plurality of optical transmitters 15 and polarization preserving optical multiplexers 16 respectively.
The light signals of odd-numbered array wavelengths λ15-1, λ15-3 . . . , λ15-(2i−1) (i is a positive integer) outputted from the optical transmitters 15-1, 15-3 . . . , 15-(2i−1) of the odd-number array polarization multiplexing portion 401 are outputted in a state of being preserved in a fixed polarization direction and are polarization-preservation-multiplexed by the polarization preserving optical multiplexer 16-1.
The light signals of even-numbered array wavelengths λ15-2, λ15-4 . . . , λ15-(2i) outputted from the optical transmitters 15-2, 15-4 . . . , 15-(2i) of the even-number array polarization multiplexing portion 402 are outputted in a state of being preserved to be orthogonal to the polarized waves of the light signals of odd-numbered array wavelengths and are polarization-preservation-multiplexed by the polarization preserving optical multiplexer 16-2.
The multiple light signals of the odd-number array polarization multiplexing portion 401 outputted from the polarization preserving optical multiplexer 16-1 and the multiple light signals of the even-number array polarization multiplexing portion 402 outputted from the polarization preserving optical multiplexer 16-2 are multiplexed by the polarization orthogonal multiplexer 17 with mutual polarized waves orthogonally preserved.
All the wavelength multiple light signals outputted from the polarization orthogonal multiplexer 17 have the optical level lowered by the insertion loss of the route of the input side amplified to a predetermined optical level and outputted by the optical amplifier 18. Thereafter, all the wavelength multiple light signals have band dispersion compensation of F [ps] performed thereto by the dispersion-compensating fiber 19, and the optical level lowered by the insertion loss of the dispersion-compensating fiber 19 is amplified to the predetermined optical level by the optical amplifier 20 and thereafter, it is outputted to the transmission line.
On the other hand, Japanese Patent Laid-Open No. 2001-203638 (hereafter, referred to as a document 1) discloses a configuration wherein polarization multiplexing light is rendered as one block, and each light signal is placed so that the wavelength spacing between the blocks becomes larger than the wavelength spacing of the light signals in each block, Japanese Patent Laid-Open No. 2001-094535 (hereafter, referred to as a document 2) discloses a configuration wherein the dispersion-compensating fiber is provided to a set of orthogonal polarization multiplexing signals, and Japanese Patent Laid-Open No. 2001-103006 (hereafter, referred to as a document 3) discloses a configuration wherein wavelength dispersion compensation is performed to each piece of wavelength light and then polarization orthogonal multiplexing is performed. In addition, a technology related to the document 3 is disclosed in Japanese Patent Laid-Open No. 9-046318 (hereafter, referred to as a document 4).
Although the technology disclosed in the document 1 provides a guard band between the blocks, it is a different invention from the present invention because it has no description of the dispersion compensation. The technology disclosed in the document 2 is in common with the aforementioned technology described in FIGS. 23 and 24 for providing the dispersion-compensating fiber to a set of orthogonal polarization multiplexing signals.
The technology disclosed in the document 3 performs wavelength dispersion compensation to each piece of wavelength light and then polarization orthogonal multiplexing is performed. Although it was mentioned earlier that “the ideal is to provide a dispersion-compensating fiber to each light signal input portion . . . . Nevertheless, such a dispersion-compensating fiber has not been developed to date,” the technology disclosed in the document 3 is supposedly an exception thereto. However, the configuration of the dispersion-compensating fiber 16-1 disclosed in the document 3 (refer to FIG. 1 of the document 3) is complicated and particular, and so it is totally different from the present invention in that it cannot divert the dispersion-compensating fiber which has been existing. The document 4 is based on the document 3.
However, there is the following problem as to the orthogonal polarization multiplexing transmission apparatus of DWDM in the past shown in FIGS. 23 and 24. To be more specific, the transmission line has dispersion inclination against the wavelength, and so a difference in cumulative dispersion between the shortest wavelength and the longest wavelength becomes larger according to a transmission distance, and a dispersion compensation limit thereof becomes a major factor of a transmission distance limit. In the case of the aforementioned method in the past, in general, a cumulative dispersion error in a zero dispersion wavelength of the transmission line is selected as a dispersion compensation value to perform collective dispersion compensation to all the wavelengths, and thus it is not possible to perform the dispersion compensation considering the short wavelength side and the long wavelength side of the zero dispersion wavelength, and consequently, only a minimum transmission distance can be secured. The above documents 1 to 4 do not disclose the means for solving this problem, either.